Summery: Nanotechnology (“Nanotech”) is a multidisciplinary field covering a vast range of devices from Engineering, Physics, Chemistry and biology. Application of Nanotech is not new to Water Treatment as the glass is Half-Full for nano-based water treatments. As Human we are thirsty species and we couldn’t survive without clean drinking water. Almost 47% of total predicted population of world has no access to safe drinking water but Nanotech can go a long way to ensure the availability of safe drinking water as it is a tiny solution to large problems. Nanotech can remove toxic metals, hazardous organic molecules, inactivate microbes (bacteria & viruses) or can turn salt water into fresh water. However this paper reviews the antimicrobial mechanism, merits, limitations and applicability of several noanoparticles such as peptides and Chitosan, TiO2, nAg, & carbon nanotubes for water disinfection.
Application of Nanotechnology (‘Nanotech’) in water treatment is not new as the glass is half-full. As human we are thirsty species and couldn’t survive without clean drinking water. Sand filtration and chlorine disinfection are effectively used in the developed world for more than a century. However there are still outbreaks of water borne diseases at high rates. Almost 47% of total predicted population of world has no access to safe drinking water. According to Centre of Disease Control Morbidity and Mortality weekly report, there were 155 outbreaks and approximately 0.4 million cases of illness in public and and U.S. water systems from 1991 to 2000 (Chemistry Division of the American Chemistry Council 2003). According to WHO report (2004), water borne diseases kill approx. 2.2 million people every year. The reason behind water borne diseases may include ineffective water disinfection as conventional water disinfection technologies (free chlorine, ozone & chloramines) have some limitations due to the formation of disinfection by-products (DBPs). More than 600 DBPs has been reported in the literature (Kranser et al., 2006). Therefore, there is a need of more innovative approach for reliable water disinfection and avoid DBPs approach.
Nanotech is rapidly enhancing field with significant applications in the water treatment. It has the capacity to revolutionize the conventional water treatment processes (USEPA, 2007). Noanoparticles are well known adsorbent, catalyst and sensors but they have shown effective antimicrobial properties including chitosan (Qi et al., 2004), silver noanoparticles (nAg) (Morones et al., 2005), photocatalytic TiO2 (Cho et al., 2005) and carbon nanotubes (CNT) (Kang et al., 2007). These noanoparticles are not strong oxidant and hence DBPs are not expected.
2. Antimicrobial Nanoparticles
Antimicrobial Nanoparticles are of three types;
- Naturally occurring antimicrobial substances
- Metal and metal oxides
- Engineered Nanoparticles
These interact with microbes through variety of mechanism as shown in figure 1. e.g. Direct interaction with microbial, interrupting electron transfer, penetrating the cell envelope, oxidizing cell components or produce secondary products (dissolved heavy metal ions) etc.
2.1. Antimicrobial Chitosan
It can be used in water processes engineering as a part of filtration process. It can be obtained from chitin in arthropod shells. It is recently been converted into Nanoparticles (Qi et al., 2004). Nano-scale Chitosan and its derivatives exhibit antimicrobial effects towards bacteria, viruses & fungi (Badawy et al., 2005). It is more effective for viruses & fungi than bacteria (more effective for gram-positive). Various mechanisms are reported for Chitosan such as (Qi et al., 2004);
- Positively charged Chitosan interacts with negatively charged cell membrane which would enhance membrane permeability causing leakage of intracellular components.
- In fungal cells, Chitosan penetrate the cell wall and bind with DNA and inhibits RNA synthesis.
Antimicrobial activities of natural & engineered Chitosan are highly affected by pH. They are not effective over pH of 6 (Qi et al., 2004).
2.2. Silver Nanoparticles (nAg)
Silver compounds and silver ions are applied in wide range of applications from disinfecting medical services and home appliances to water treatment. However the mechanism is still partially understood (Chou et al., 2005). Silver ion interacts with proteins in microbial cell to inactivate respiratory enzymes and produce reactive oxygen species (ROS). Literature shows that Ag+ also prevents DNA replication. The ability of UV radiation in the presence of Ag ion enhances to inactivate bacteria & viruses (Kim et al., 2008). The mechanism of nAg is summarized as under (Qilin Li et al., 2008);
- Adhesion of Nanoparticles to the surface altering the membrane properties such as permeability.
- nAg penetrate into the bacterial cell, resulting in DNA damage.
- Dissolution of nAg to release antimicrobial Ag ion.
2.3. TiO2 Nanoparticles
It is the most commonly used and most studied. It is activated by UV-A radiation or visible light (sun light) and can be used to remove contaminants from water and air. It can kill both gram positive and negative bacteria. More recently, nano-sized TiO2 is reported to kill viruses (polioviruses & hepatitis B virus) (Qilin Li et al., 2008). Antimicrobial activity of TiO2 is related to ROS production. Recently it was demonstrated that doping of TiO2 with silver greatly improves the photolytic inactivation of bacteria and viruses (Page et al., 2007).
2.4. Carbon Nanotubes
Carbon Nanotubes are the sheets rolled in to tubes. They can be either single walled (SWNTs) with diameter 1 – 5 nm, or multi-walled (MWNTs) with diameter up to 100nm. The effectiveness of CNTs is decreasing from SWNTs>MWNTs>quartz>C60 (Qilin Li et al., 2008). Surprisingly, CNTs has got no particular attention due to its disbursement in water. Few studies available credited SWNT swith antimicrobial activity towards bacteria and the mechanism may be either physical interaction or oxidative stress that affects cell membrane. Therefore, CNTs may inhibit microbial attachment & bio-fouling formation (Kang et al., 2007).
3. Disinfection & Microbial Control by Nanotech – Current Status
Mechanism of various nano-particles is explained above. These particles have various applications in diverse products including water treatment.
- Silver nano-particles are most commonly (almost 100 consumer products) used e.g. nutrition supplement, kitchen appliances & home purification system (Maynard 2007).
- Chitosan nano-particles have applications in cosmetic and food preservation products and can be used as coagulant in water/wastewater treatment (Zhang et. al., 2008).
Table 1 below is showing the current potential application of nano-particles.
Table 1: Current & Future Application of Nano-Particles (Qilin Li, 2008)
|Nanoparticles||Current Applications||Future Applications||Antimicrobial Mechanism|
|Bio – sorbent|
|Silver||Portable Water filters|
|Membrane surface coating||Membrane damage
Release of Ag+
|TiO2||Organic degradation in Water treatment plant |
|Bio-fouling resistant surface Reactive membrane||Cell wall and membrane damage
Production of ROS
|Carbon Nanotubes||-||Carbon hollow fiber|
Pack bed reactors
|Physical interaction with bacteria|
4. Effectiveness of Disinfectants
There are various factors affecting the effectiveness of various disinfectants as shown in table 2 below and table 3 would present relative effectiveness of various disinfectants.
Table 2: Factors Affecting the Effectiveness of Disinfectants
|Sr. No.||Influencing Factors||Comments|
|1.||Contact time and Dose Time||It can be determined by Chick’s Law & Watson’s Law.|
|2.||Water Quality Characteristics||pH, turbidity, organic compounds, temperature and heavy metals|
|3.||disinfection byproducts (DBPs)||Nano-particles do not produce any DBPs.|
|4.||Complexity of Process||More complex system would be less effective.|
Table 3: Relative Effectiveness of Various Disinfectants (Qilin Li, 2008)
|Sr. No ||Disinfectants||Watson’s Law (CT)|
|1.||Chitosan||7.5 – 144||At minimum inhibition concentration|
|3.||Silver Ion||0.075 – 26||For 99% disinfection of Pseudomonas Aeroginosa|
|4.||Chloramines||95 – 180|
|5.||Free Chlorine||0.03 – 0.05|
|6.||UV-C (254 nm)||3.4 – 4.8|
|7.||TiO2 + UV (300 – 400nm )||3600 – 8500||TiO2 slurry in UV reactor|
5. Combination of Current Technologies with Nanotech
Nanotechnology is being used in combination with conventional technologies as hybrid process. A most typical example is the use of UV reactors + photosensitive nano-particles. UV reactors are most commonly used in developed countries due to its effectiveness against Giardia & Cryptosporidium but not very effective against some pathogenic viruses such as adenoviruses (Yates et al., 2006). Therefore, UV reactor in combination with photosensitive nano-particles provides additional inhabitation mechanism. TiO2 is coated on UV reactor as TiO2 also have the capability to degrade organic contaminants & natural organics.
Nano-particles have applications in membranes; rapidly growing technique in water & wastewater treatment but the fouling of membrane materials is significant barrier in obtaining the desired efficiency. Photoactive nano-particles make the membranes reactive and thus the membrane would not remain as physical barrier. Membrane with TiO2 coating in the presence of UV-A radiation inhibits bacteria, organic matter and reduce bio-fouling of membrane (Choi et al., 2007). Similarly, silver coated hollow fibres have enhanced anti-bacterial activity against E-coli & S. aureus (Qilin Li, 2008).
6. Technological Challanges
Application of antimicrobial Nano-particles is still developing in water treatment due to dispersion, retention and sustainability of antimicrobial activity of nano-particles.
- TiO2 combined severely when added to water but very stable in pure water.
- Nanoparticles are of very small size in suspension, thus there would be the requirement of efficient removal process like membrane filtration.
- Nano-particles should be used in combination with secondary disinfectant as all the nano-particles are separated from product water and thus leaving no residual.
- Some nano-particles have negative impacts on human health. Long term exposure to high concentration Ag+ may cause darkening of skin. nC60 may harm mammalian cells. Thus separation of nano-particles from product water is necessary.
- Overall, the discussed nano-particles are promising disinfectants such as nAg, Chitosan, CNT, TiO2.
- Most important benefit of nano-particles are the non-generation of Disinfection By-products (DBPs).
- There is still lot of research work is required especially for the separation of nano-particles from the product water as the separation process would enhance the cost of the process.
- Efforts may be required to use antimicrobial Nanoparticles in conjunction with conventional treatment processes.
- The paper reviewed is originated from the application of nano-particles on simple aqueous solution. Their behaviour in complex system is yet to be determined.
- The actual benefit of conventional disinfectants is low cost, thus more low cost anti-microbial disinfectant must be explored.
- Badawy, M.E.I., Rabea, E.I., Rogge, T.M., Stevens, C.V., Steurbaut, W., Hofte, M., Smagghe, G., 2005. Fungicidal and insecticidal activity of O-acyl chitosan derivatives. Polymer Bull. 54 (4–5), 279–289.
- Chlorine Chemistry Division of the American Chemistry Council, 2003. Drinking Water Chlorination: A Review of Disinfection Practices and Issues [cited 2008 April 10]; Available from: <http://www.c3.org/chlorine_issues/disinfection/c3white2003.html>.
- Cho, M., Chung, H., Choi, W., Yoon, J., 2005. Different inactivation behavior of MS-2 phage and Escherichia coli in TiO2 photocatalytic disinfection. Appl. Environ. Microbiol. 71 (1), 270–275.
- Choi, H., Stathatos, E., Dionysiou, D., 2007. Photocatalytic TiO2 films and membranes for the development of efficient wastewater treatment and reuse systems. Desalination 202, 199–206.
- Chou, W.L., Yu, D.G., Yang, M.C., 2005. The preparation and characterization of silver-loading cellulose acetate hollow fiber membrane for water treatment. Polymer. Adv. Tech. 16 (8), 600–607.
- Kang, S., Pinault, M., Pfefferle, L.D., Elimelech, M., 2007. Singlewalled carbon nanotubes exhibit strong antimicrobial activity. Langmuir 23, 8670–8673.
- Kim, J.Y., Lee, C., Cho, M., Yoon, J., 2008. Enhanced inactivation of E. coli and MS-2 phage by silver ions combined with UV-A and visible light irradiation. Water Res. 42 (1–2), 356–362.
- Krasner, S.W., Weinberg, H.S., Richardson, S.D., Pastor, S.J., Chinn, R., Sclimenti, M.J., Onstad, G.D., Thruston Jr., A.D., 2006. Occurrence of a new generation of disinfection byproducts. Environ. Sci. Technol. 40, 7175–7185.
- Maynard, A.D., 2007. Nanotechnology – toxicological issues and environmental safety and environmental safety. In: Project on Emerging Nanotechnologies, 1–14. Woodrow Wilson International Center for Scholars, Washington, DC.
- Morones, J.R., Elechiguerra, J.L., Camacho, A., Holt, K., Kouri, J. B., Ramirez, J.T., Yacaman, M.J., 2005. The bactericidal effect of silver nanoparticles. Nanotechnology 16 (10), 2346–2353.
- Page, K., Palgrave, R.G., Parkin, I.P., Wilson, M., Savin, S.L.P., Chadwick, A.V., 2007. Titania and silver-titania composite films on glass-potent antimicrobial coatings. J. Mat. Chem. 17 (1), 95–104.
- Qi, L., Xu, Z., Jiang, X., Hu, C., Zou, X., 2004. Preparation and antibacterial activity of chitosan nanoparticles. Carbohydr. Res. 339 (16), 2693–2700.
- Qilin Li, Shaily Mahendra, Delina Y. Lyon, Lena Brunet, Michael V. Liga, Dong Li, Pedro J.J. Alvarez, 2008, Antimicrobial nanomaterials for water disinfection and microbial control: Potential applications and implications. Water research (42), 4592 – 4599.
- USEPA, 2007. In: Science Policy Council (Ed.), US Environmental Protection Agency nanotechnology white paper. EPA 100/B-07/001 Washington, DC.
- WHO (2004) available at: http://www.who.int/water_sanitation_health/en/factsfigures04.pdf,
- Yates, M.V., Malley, J., Rochelle, P., Hoffman, R., 2006. Effect of adenovirus resistance on UV disinfection requirements: a report on the state of adenovirus science. J. Amer. Water Works Assoc. 98 (6), 93–106.
- Zhang, B., Fortner, J.D., Lee, J., Huang, C.-H., Kim, J., Hughes, J.B., 2008. Catalytic degradation of indigo dye by aqueous stable C60 aggregates. In: 235 American Chemical Society National Meeting & Exposition New Orleans, LA.